For a number of years, telephone companies have been providing a service which assigned two or more directory numbers per line and corresponding distinctive ringing signals. One of the telephone switch vendors refer to this feature as "Multiple Directory Numbers per Line" or "MDNL," and the local telephone companies have offered the service under a variety of names such as "IdentaRing.TM.". With such a service, the telephone central office switch connected to a called subscriber's line will apply a different ringing signal to the line depending on which of the assigned directory numbers the current caller dialed. For example, if a caller dialed the main number assigned to the subscriber, the local telephone office switch may apply a standard ringing signal (two seconds on, four seconds off) to the called subscriber's line. If the caller dialed the second number assigned to the subscriber's line, the local telephone office switch applies a different ringing signal to the line.
The various ringing signals produce different audible rings at the subscriber's premises. As a result, persons present at the subscriber's premises can tell which number the caller dialed before answering each incoming call. The ability to identify the dialed number allows the subscriber and persons at the subscriber's premises to give the numbers to different potential callers for a variety of different purposes.
For example, if a family subscribes to the service and the telephone company provides the service via the family's residential line, the main number might be used for the parents and/or for general family activities. The secondary number, however, could be given to a teenage member of the family. Calls producing an audible ring corresponding to the secondary number would indicate in advance that an incoming call was for the teenager. In such a case, the parents might choose not to answer the call. Similarly, if a subscriber operates a business out of the home, one of the numbers and corresponding ringing sounds might be used for incoming business calls.
Another telephone service which has recently become quite popular is voice mail. With a typical voice mail service, such as those offered through local telephone companies, a caller dials the subscriber's telephone number. If the subscriber's line identified by the subscriber's "mailbox" telephone number is busy or if there is no answer, the telephone network forwards the call to a voice mail system. This voice mail system provides an answering prompt message to the caller. In many systems, this prompt will include a personalized message recorded by the subscriber, for example the subscriber's name spoken in the subscriber's own voice. After the prompt, the voice mail system records a message from the caller in the called subscriber's mailbox.
Such voice mail systems typically service a large number of telephone service subscribers. The telephone system forwards data to the voice mail system identifying the telephone line or station called. The voice mail system uses this data to retrieve the called subscriber's personalized prompt and to store incoming messages in the correct subscriber's mailbox. Typically, the telephone system forwards the telephone number of the called line to the voice mail service. Problems have arisen, however, where the called subscriber also subscribes to a multiple directory number per line type service such as "IdentaRing.TM.".
In prior art systems, the type of telephone central office switch to which the called subscriber's line connected determined how the system would treat calls to subscriber's of both voice mail and a multiple directory number per line type service. If the switch was the product of one manufacturer, it might forward calls for the main number to the voice mail system but block forwarding of calls when the caller dialed one of the secondary numbers.
Switches from other vendors would always forward the call and transmit the actual number dialed to the voice mail system for identification purposes. If the subscriber subscribed to a different mailbox for each of the numbers, this would allow proper routing to the individual mailboxes. If the subscriber chose to subscribe to only a single mailbox, however, the voice mail system could not process incoming calls identified by numbers other than the main number, which typically corresponded to the single mailbox.
Conversely, other switches would forward calls but would only transmit the main number, corresponding to the line of the called subscriber. This number would be sufficient if the subscriber has only one mailbox. If the subscriber has multiple mailboxes, however, there was no effective way to route incoming calls to the different mailboxes. To approximate different mailboxes, the voice mail system could be programmed to offer the single mailbox in partitioned or subdivided form. To reach a subdivided section within the mailbox, a caller would have to listen to a general prompt message and select the proper sub-mailbox, e.g. for the family in general, for one or both of the parents, for the teenager, or for the business operated from the home. The requirement to select which sub-mailbox places an additional burden on callers and may discourage some callers from leaving a message. Also, delivery of only the main number could not be used to route messages to any particular subsection within the mailbox without the manual selection procedure.
From the above discussion it becomes clear that existing systems do not provide sufficient flexibility of operation for effective service to subscribers to both multiple directory number per line type services and message service systems, such as voice mail services.